A vaccine pill that protects the body against the allergic effects of grass pollen will be made available to around one million hay fever sufferers in Britain from next month. The pill, which contains tiny amounts of grass pollen, will be offered on prescription to hayfever sufferers who do not respond well to existing antihistamines and nasal steroid sprays.

Called Grazax, the pill dissolves under the tongue and, to be effective, must be taken eight weeks before the hayfever season begins. Trials with the pill followed successful tests of a grass pollen injection, which was found to protect hayfever sufferers from pollen allergy for three years after their treatment was stopped.

Hay fever affects around 15 million people in Britain, but the number is rising. Scientists are uncertain why, but leading theories include dietary changes, such as a drop in antioxidants in modern diets, and a lack of early exposure to allergic substances. The hay fever season is also expanding, as warmer springs and milder autumns stretch the pollen season to run from March to early August. Grass pollen is the main culprit, affecting 95% of sufferers. Unlike flowering plants which are pollinated by insects, grasses, weeds and trees produce billions of ultralight pollen particles that have evolved to hang in the air and spread as far as possible.

Studies with the new pill, which contains Timothy grass, suggest it effectively primes the immune system not to over-react to pollen and cause hay fever. In trials volunteers taking the pill reported on average a 30% improvement in symptoms such as itchy, streaming eyes, fatigue and sneezing. Patients also reported a 10% to 20% improvement after taking antihistamines and a 20% to 30% improvement with steroidal nasal sprays.

"Initially, this will be aimed at people for whom other treatments are not effective," said Stephen Durham, an allergy specialist at the National Heart and Lung Institute at Imperial College London, who is investigating the long-term effects of Grazax. "I believe about 10% of the hay fever population, potentially a million patients in the UK, could benefit from this treatment."

Prof Durham's team at the MRC-Asthma UK Centre in Allergic Mechanisms of Asthma, based at King's College and Imperial College, London, is trialling the pill to see if it has the same kind of long-term benefit as injected vaccines. He said the cost of the treatment is expected to be "hundreds of pounds" a year.

The pill joins a highly competitive market. Hay fever remedies are now the fastest growing over-the-counter medicine in the country, and companies in the market took more than £80m from the British public this year.

Grazax, made by the drug company Alk-Abello, has been approved for use in 27 European countries, including the UK, and has been introduced in Sweden. While studies with injectable grass pollen vaccines found them to be effective if given once a month, a daily pill is believed to be more appealing to patients, said Prof Durham. Side-effects included occasional itching or slight swelling under the tongue where the pill is placed until it dissolves.